1. Field of the Invention
The present invention relates to a method of making molds which comprises forming a thin sheet of facing sand having long-lasting plasticity, applying said sheet to the surface of a pattern, and charging backing sand to fill the cavity.
2. Description of the Prior Art
Generally, for facing sand, use is made of sand of fine particle size so as to attain a dense and smooth casting surface, said sand being mixed with an inorganic binder, such as sodium silicate and bentonite, or an organic resin binder, and an indefinte amount of such facing sand is manually applied to the surface of a pattern while being pressed thereagainst and backing sand is then charged to fill the cavity while being manually or mechnically rammed around, whereupon the pattern is removed to leave a mold.
Thus, according to the conventional method as described above, the amount of facing sand used is indefinite and the thickness of the layer of facing sand in the various portions is ununiform to the extent that the backing sand is sometimes exposed to the surface of the mold.
Therefore, the conventional method requires that the thickness of the layer of facing sand be at least 20 mm and usually 30 to 50 mm and in some cases exceed 100 mm.
The use of such a large amount of facing sand is disadvantageous in that it correspondingly increases the cost of making molds.
Since the facing sand is small in particle size, the increased thickness of the layer of facing sand will impede the dissipation of gases produced during the pouring of molten metal, resulting in a cause of casting defects.
Further, the increased thickness of the layer of facing sand will greately aggravate the dissipation of heat after casting, thus increasing the amount of sand burning, which, in turn, requires much time and labor for sand stripping subsequent to the opening of the mold.
On the other hand, the nonuniform thickness of the layer of facing sand results in the nonuniformity of the pressure-resisting strength to withstand the metal pouring pressure and also results in the nonuniformity of heat expansion, causing defects in the casting surface.
Further, the bentonite used in the conventional facing sand is a kind of clay which has a disadvantage that when dried, it loses its binding force, so that the layer of facing sand tends to collapse.
Further, sodium silicate and resin binders, such as furan, have self-curing property and will cure in a short time, thus having a disadvantage that it is impossible to retain plasticity for a long time.